The present invention relates to the preparation of methylpolysilanes, and more particularly to a method of preparing methylpolysilanes by a catalyzed redistribution of alkoxydisilanes, and the methylpolysilanes produced thereby.
In recent years, workers in the art have developed procedures for the preparation of silicon carbide ceramic materials from polymeric silane precursors such as methylpolysilanes. Silicon carbide possesses a number of desirable properties such as chemical inertness, semiconducting properties, extreme hardness and stability at very high temperatures. Accordingly, silicon carbide ceramics have found use in electrical heating units, furnace walls, mufflers, abrasives, rocket nozzles, and automotive and turbine engine parts. Further, it has been found that the use of polymeric precursors permits the formation of fibers and thin films or coatings of silicon carbide which were heretofore extremely difficult to form using inorganic sources of silicon carbide.
Baney et al, U.S. Pat. No. 4,310,651, teach a procedure for the preparation of methylpolysilanes having halogen substituents through a catalyzed redistribution reaction utilizing tetrabutylphosphonium chloride as the catalyst. The Baney et al process has the advantage of being able to utilize as a starting material the process residue from the direct synthesis of organochlorosilanes. Direct synthesis of organochlorosilanes involves passing the vapor of an organic chloride over heated silicon and a catalyst. See, Eaborn, Organosilicon Compounds, Butterworths Scientific Publications, 1960, page 1. This residue contains a mixture of di-, tri-, and tetra-substituted halodisilanes. However, the halogen substituents on the methylpolysilanes of the Baney et al process have resulted in some difficulties in handling the compositions which tend to auto-ignite when exposed to oxygen or moisture. Moreover, pyrolysis of the compositions to form ceramics releases large quantities of corrosive HCl or HBr gases which must be handled and properly disposed of.
Baney et al, U.S. Pat. No. 4,298,558, teach an improved procedure which converts the halogen substituents on the methylpolysilanes to alkoxy or phenoxy substituents. However, the improved procedure still requires a two step process of converting halodisilanes to halo-substituted methylpolysilanes and then converting the halogen substituents to alkoxy or phenoxy-substituted compositions.
Other workers have attempted to produce methylpolysilanes by a single step redistribution reaction using methoxydisilane starting materials. For example, Ryan et al, 84 J. Amer. Chem. Soc. 4730 (1962), reported the redistribution of 1,1,2,2-tetramethoxy-1, 2-dimethyldisilane to higher polysilanes in the presence of sodium metal. Watanabe et al, in a series of published reports, taught that metal alkoxide catalysts could be used in the redistribution reaction. See, e.g., Watanabe et al, J. C. S. Chem Comm. (1977) 534; Watanabe et al, J. C. S. Chem. Comm. (1977) 704; Watanabe et al, 128 J. Organometallic Chem. 173 (1977); Watanabe et al, J. C. S. Chem. Comm., (1978) 1029; Watanabe et al, 218 J. Organometallic Chem. 27 (1981); and Watanabe et al, 244 J. Organometallic Chem. 329 (1983).
Atwell et al, 7 J. Organometallic Chem. 71 (1967), have also reported the redistribution of alkoxy disilanes to higher organopolysilanes. However, in the Watanabe and Atwell reports, the higher organopolysilane was either uncharacterized, unidentified, or was of a low molecular weight (less than 6 silicon atoms in the chain).
More recently, Frey et al, U.S. Pat. No. 4,667,046, teach a method for preparing higher molecular weight methylpolysilanes by reacting a trialkoxy-substituted disilane, and optionally a tetraalkoxy-substituted disilane, with a silane having at least one silicon to hydrogen bond in the presence of an alkali metal alkoxide catalyst. The methylpolysilanes are taught to be useful as negative photoresist coatings and ceramic precursors. However, the starting materials of Frey must be separately prepared. Frey does not teach the ability to use the process residue from the direct synthesis process which contains mixtures of di-, tri-, and tetra-substituted materials.
Accordingly, the need still exists in the art for a relatively simple single step process for the production of higher molecular weight methylpolysilanes which are free of halogen substituents. Further, there is still a need in the art for such a process in which readily available, inexpensive starting materials containing difunctional substituted disilanes can be utilized.